This invention relates to an antenna apparatus for use in a digital radio receiver and, in particular, to an antenna apparatus for receiving both of a radio wave from an artificial satellite and a radio wave from a ground station.
At present, digital radio broadcasting (at a frequency of about 2.3 GHz) utilizing an artificial satellite (broadcasting satellite) is about to start in the United States of America. In this situation, development is made of digital radio receivers for receiving the digital radio broadcasting.
The digital radio receivers are classified into two types, one of which is adapted to directly receive a radio wave (may be called xe2x80x9csatellite wavexe2x80x9d hereinafter) transmitted from the artificial satellite and the other of which is adapted to receive a frequency-shifted radio wave (may be called xe2x80x9cground wavexe2x80x9d hereinafter) broadcasted from a ground station where the radio wave from the artificial satellite is received and shifted in frequency to produce the frequency-shifted radio wave.
The digital radio receiver of the first type for directly receiving the satellite wave is intended to be mounted on a mobile body such as an automobile. Since the satellite wave is susceptible to the weather, it is desired that the digital radio receiver mounted on the mobile body can receive not only the satellite wave but also the ground wave.
However, the satellite wave is a circular polarized wave (or a circular polarization) while the ground wave is a linear polarized wave (or a polarization). Accordingly, in order to receive both of the satellite wave and the ground wave, special-purpose reception antennas are required to receive the satellite wave and the ground wave, respectively.
An existing satellite-wave antenna apparatus for receiving the satellite wave comprises an antenna such as a helical antenna, a low-noise amplifier connected to the antenna, and a case accommodating the antenna and the low-noise amplifier. Similarly, an existing ground-wave antenna apparatus for receiving the ground wave comprises an antenna such as a monopole antenna, a low-noise amplifier connected to the antenna, and a case accommodating the antenna and the low-noise amplifier.
Thus, the existing satellite-wave antenna apparatus and the ground-wave antenna apparatus are independent of each other. Therefore, a combination of the satellite-wave and the ground-wave antenna apparatuses requires a large number of components to be assembled and much labor and time in assembling these components. This inevitably results in an increase in cost. In addition, such combination of the satellite-wave and the ground-wave antenna apparatuses requires a large space for installation.
It is therefore an object of this invention to provide an antenna apparatus which is capable of receiving both a satellite wave and a ground wave and which has a simplified structure.
It is another object of this invention to provide an antenna apparatus which is capable of receiving both a satellite wave and a ground wave and which requires a less number of components and is easy in assembling.
It is still another object of this invention to provide an antenna apparatus which is capable of receiving both a satellite wave and a ground wave and which is small in size.
Other objects of this invention will become clear as the description proceeds.
According to a first aspect of this invention, an antenna apparatus comprises a plurality of antennas for individually receiving, as reception signals, radio waves different from one another in frequency and is connected to a receiver body. The antenna apparatus comprises a single case or a single substrate to which the antennas are mounted, and a single cable for transmitting to the receiver body a combined reception signal obtained by combining the reception signals received by the antennas.
According to a second aspect of this invention, an antenna apparatus comprises a helical antenna. The helical antenna comprises an insulating cylindrical bobbin, a wire helically wound around an outer peripheral surface of the cylindrical bobbin, and a plurality of ring-shaped insulating wire holders fitted to the cylindrical bobbin to fix the wire to the cylindrical bobbin.
According to a third aspect of this invention, the antenna apparatus comprises a helical antenna and a top cover. The helical antenna comprises a plurality of wire holders. An uppermost one of the wire holders has a recess or a cut formed at its outer peripheral portion. The top cover is provided with a protrusion to be engaged with the recess or the cut so as to inhibit the rotation of the uppermost wire holder in a circumferential direction of the helical antenna.
According to a fourth aspect of this invention, the antenna apparatus comprises a plurality of antennas each of which is provided with a boss pin formed at its lower end, and a case or a substrate to which the antennas are mounted. The boss pin is inserted into a hole formed in the case or the substrate and is fused to the case or the substrate.
According to a fifth aspect of this invention, the antenna apparatus comprises a helical antenna having a cylindrical bobbin, and a monopole antenna arranged inside the cylindrical bobbin. The cylindrical bobbin comprises an outer cylindrical member, a hollow center member having a center axis coincident with that of the outer cylindrical member and adapted to receive the monopole antenna to be inserted therein, and at least three ribs radially extending from the center member to the outer cylindrical member to connect the center member and the outer cylindrical member. The outer cylindrical member, the center member, and the ribs are integrally formed.